Field of the Invention
The present invention relates to an optical waveguide device, and in particular, relates to an optical waveguide device which is able to efficiently remove stray light propagating through a substrate.
Description of Related Art
In optical communication or optical information processing, various optical waveguide devices such as an optical modulator are used. A waveguide type LN modulator using a substrate of lithium niobate (LN) or the like having an electro-optic effect has a small wavelength chirp and is able to perform phase and intensity modulation, and thus is used as a multi-level modulator such as a DQPSK modulator, a DP-QPSK modulator, and a QAM modulator.
In an intensity modulator of the related art, a driving voltage applies a voltage (Vπ) changing from an On state to an Off state, but in an optical modulator such as the multi-level modulator using an optical phase, it is necessary to modulate the phase from π to −π, and 2Vπ of the driving voltage is necessary. In order to reduce a load of a drive circuit (a driver) of the optical modulator, it is necessary to decrease the driving voltage of the optical modulator to half.
In addition, a changeover from intensity modulation to multi-level modulation is the purpose to increase transmission capacity, and thus it is necessary to decrease the driving voltage without degrading optical (response) bandwidth. However, the driving voltage and the optical bandwidth have a contradictory relationship (refer to Japanese Examined Patent Application Publication No. H7-50265), and thus even if a technology of the related art is used, it is difficult to realize a decrease in the driving voltage and an increase in the bandwidth.
As a method of realizing a decrease in the driving voltage and an increase in the bandwidth, a method of forming a ridge structure on the surface of an LN substrate, or a method of thinning the LN substrate is proposed.
When the thickness of the LN substrate is thinned to the extent of a light distribution, a microwave electric field and an optical waveguide efficiently overlap each other, and thus it is possible to decrease the driving voltage. In addition, it is possible to decrease a dielectric loss cursed by LN, and thus it is possible to improve the optical bandwidth by reducing a microwave loss.
On the other hand, leaked light from an optical connection portion between a fiber and an LN chip configuring an optical waveguide device or an optical waveguide, and unnecessary light such as radiation light radiated from a Y-junction of the optical waveguide at the state of extinction, or the like propagates through the LN substrate. The LN substrate is thinned, and a cross-sectional area of the LN substrate decreases, and thus a peak of the optical intensity becomes larger. That is, it is known that it makes difficult to control a bias point of the optical modulator because the extinction ratio caused by recombining stray light to optical waveguide is degraded caused by increasing the average optical intensity of stray light or the like against the optical intensity of light wave propagating through the substrate or the stray light is mixed into a photo detector (PD) for control.
In the optical modulator having a thin substrate structure, as a method of removing the stray light (the leaked light), the following technologies are proposed.
(1) An optical absorption material (metal) is arranged in a region other than the vicinity of the optical waveguide (refer to Japanese Laid-open Patent Publication No. 2006-276518, Japanese Laid-open Patent Publication No. 2006-301612, and Japanese Laid-open Patent Publication No. 2012-078507).
(2) A through hole is formed in the thinned LN substrate (a clad portion) (refer to Japanese Laid-open Patent Publication No. 2006-276518 and Japanese Laid-open Patent Publication No. 2006-301612).
(3) A 3-branch structure or a residual high order mode removing function is applied to the Y-junction of the optical waveguide (refer to Japanese Laid-open Patent Publication No. 2011-075906).
(4) The 3-branch structure is applied to the Y-junction of the optical waveguide, and a method of guiding light propagating through a subsidiary waveguide to the outside is adopted (refer to Japanese Laid-open Patent Publication No. 2012-078508 and Japanese Laid-open Patent Publication No. 2012-215901).
(5) A method of terminating (absorbing or ejecting) the leaked light from a branched point of a Y-branched portion of the optical waveguide is adopted (refer to Japanese Patent Application No. 2012-176628 (Filing Date: Aug. 9, 2012)).
Next, a forming method of the LN modulator using the thin substrate of the technologies of the related art and metal absorption of the leaked light will be described.
The forming method of the LN modulator using the thin substrate is as follows.
A back surface of an X-cut LN wafer on which a Ti diffused waveguide is formed is thinned to approximately 10 μm by a polishing equipment (CMP) or the like. In a Ti diffusion step, thin substrate processing, or the like, a conventional technology is able to be used.
An electrode of the optical modulator may be formed before the LN substrate is thinned or may be formed after the LN substrate is thinned. In addition, when only an interaction area of the electrode is thinned, masking is formed on the back surface of the substrate, and then the thinning is able to be performed by a wet etching method, a dry etching method, a sandblasting method, or further more laser processing using an excimer laser or the like.
The thinned LN wafer is fixed to a holding substrate with an adhesive agent. After that, in order to form the electrode by using electroplating, a plating seed layer is formed on the surface of the thinned LN substrate through a bonding layer by using vacuum vapor deposition or the like. It is necessary that the electrode of the optical modulator adheres by metal to a dielectric material or a semiconductor. In addition, it is required that a material of the electrode is a material by which the electrode is not deformed (Migration) due to apply electric current and does not cause a solid phase alloy reaction with other materials or the like.
For this reason, a two-layer structure having the bonding layer between the electrode and the LN substrate is used. In the bonding layer (a metal layer), titanium, chromium, or the like which is disclosed in Japanese Patent No. 3628342 is used. That is, when the electrode material is gold, titanium and gold, chromium and gold, and the like are included therein, and when the electrode material is copper, titanium and copper, chromium and copper, and the like are included therein. After the seed layer is formed, the electrode is formed by a semi-additive method using electroplating. A cross-sectional shape of the LN modulator having a thin substrate structure is illustrated in FIG. 1.
A method of absorbing the leaked light propagating through the substrate by metal will be described. The leaked light from the optical waveguide slab-propagates in the LN substrate (the clad portion). This phenomenon particularly remarkably occurs in the thinned substrate. In the past, a minimum extinction ratio is ensured by a method of absorbing the slab-propagated light using a part of the electrode configuring the optical modulator, or a method of disposing a dedicated guide (a waveguide or the like) for the leaked light and guiding the leaked light to a portion under the electrode.
On the other hand, when the optical modulator is changed from the intensity modulator to the multi-level modulator, inside the LN modulator chip using the thin substrate, a light leaked portion (for example, as the light leaked portion, the optical connection portion (connection between the chip and the fiber, or the like), a bending waveguide (in particular, when a curvature radius is small), the Y-branched portion, and the Y-junction (in principle, leaked light occurs) are included.) increases, and thus the extinction ratio is more easily degraded. In addition, when multi-level is used, a high extinction ratio greater than or equal to that of the intensity modulator is required, and thus it is not able to support to this requirement using only the conventional technology.
A result of preliminarily calculating the amount of light absorption by using the electrode (the bonding layer or the metal layer) in the light slab-propagating through the thinned LN substrate is illustrated in FIG. 2. In FIG. 2, the thickness of the LN substrate is 9.0 μm, the bonding layer (the metal layer) is made of titanium (Ti: a refractive index n and an extinction coefficient k with respect to the light waves of 1.55 μM are n=4.05358 and k=3.81057), and the electrode material is gold (Similarly, Au: n=0.56043 and k=11.2475).
The light slab-propagating in the LN substrate is in contact with the titanium of the bonding layer, and thus a relationship between the thickness of a titanium film and a propagation loss of the slab-propagated light becomes important. In addition, the LN substrate is rather thick as the optical waveguide and functions as a multi-mode waveguide, and a propagation loss of low order mode light (λ=1.55 μm, and a TE mode) is illustrated in the graph. In a 0-order mode (n=0), a hopping in the graph of the titanium film at a thickness of 0.11 μm occurs due to changing the mode of the LN substrate to a mode having the titanium film as a core. That is, the electric field profile of the 0-order mode (n=0) is shifted in the titanium film. Similarly, in a first mode (n=1), one peak of the electric field profile penetrates into the titanium film, and the other peak is in the LN substrate.
It is understood that higher order mode light has larger amount of light away from the LN substrate so that it increases propagation loss. On the other hand, the thickness of the titanium film is set to be 0.07 μm or more, and thus in a mode where the number of peaks of the electric field profile in the LN substrate is one, a metal absorption loss (the propagation loss) of 4 dB/cm is able to be obtained. The metal absorption loss is a constant value even when the thickness of the titanium film is 0.2 μm or more. In the metal absorption performed by using the titanium film, using only the metal absorption of the titanium film or the like, it is not possible to sufficiently respond to a situation where a great deal of leaked light is generated due to a complicated optical waveguide such as the multi-level modulator.
On the other hand, the light absorption using metal is used as a polarizer. In particular, a polarizer is developed in which a metal film is disposed onto the optical waveguide directly or through a low refractive index film, and thus light of a TM mode is absorbed. However, in the absorption by metal in this method, it is known that light of the TE mode is not likely to be absorbed. For this reason, a TE mode absorption type polarizer is developed in which a high refractive index film is inserted between the optical waveguide and the metal film (refer to H. A. JAMID et al., “TM-PASS POLARISER USING METAL-CLAD WAVEGUIDE WITH INDEX BUFFER LAYER”, Electronics Letters, Vol. 24, No. 4, pp 229-230, February 18 (1988), and Japanese Patent No. 2641238). However, when the configuration of this polarizer is applied to the LN substrate, it makes the problem that an insertion loss increases.
In order to achieve both a low insertion loss and absorption of unnecessary polarized light, a method of inserting a low refractive index thin film between the high refractive index film and the substrate is proposed (refer to Guangyuan Li et al., “Analysis of the TE-Pass or TM-Pass Metal-Clad Polarizer With a Resonant Buffer Layer”, Journal of Lightwave Technology, Vol. 26, No. 10, pp 1234-1241, May 15 (2008)).
In addition, in an integrated modulator such as a DP-QPSK modulator, downsizing of the entire chip is required. In order to downsize the chip, it is necessary to make a curvature radius of a bending portion or the Y-branched portion of the optical waveguide shorter. When the curvature radius makes shorter, a refractive index of the waveguide is drastically changed against a light propagation direction, and thus the leaked light increases, and it is necessary to further remove the stray light or the like.
In addition to the light absorption by metal, there is a method of forming a through hole in the substrate described above, but the chip is easily broken, and reliability as an optical waveguide device is degraded. In addition, when the optical waveguide (including the high refractive index film) guiding the leaked light is disposed, a processing method of termination of the optical waveguide becomes problematic. Obviously, it is not possible to guide the entirety of the leaked light with the optical waveguide.
In the light absorption by metal, a case where the polarization of the light waves input to the LN modulator is TE mode, it comes up more difficult problem. That is, an absorption effect of the TE mode light by the metal film is lower than an absorption effect of the light in the TM mode by approximately double digits. It is necessary that the minimum extinction ratio of a commercially available optical modulator is generally 25 dB or more. In contrast, considering the length of a Y multiplexing and demultiplexing portion of the chip in the LN modulator is approximately 1 cm, it is not possible to sufficiently perform the light absorption using only the metal film.